The present invention relates to press formable, high strength hot rolled and cold rolled steel sheets having high flow stress during dynamic deformation, which can be used for automobile members and the like to provide assurance of safety for passengers by efficiently absorbing the impact energy of a collision, as well as a method for producing the same.
In recent years, protection of passengers from automobile collisions has been acknowledged as an aspect of utmost importance for automobiles, and hopes are increasing for suitable materials exhibiting excellent high-speed deformation resistance. For example, by applying such materials to front side members of automobiles, the energy of frontal collisions may be absorbed as the materials are crushed, thus alleviating the impact on passengers.
Since the strain rate for deformation undergone by each section of an automobile upon collision reaches about 103 (l/s), consideration of the impact absorption performance of a material requires knowledge of its dynamic deformation properties in a high strain rate range. Because it is also essential to consider at the same time such factors as energy savings and CO2 exhaust reduction, as well as weight reduction of the automobile, requirements for effective high-strength steel sheets are therefore increasing.
For example, in CAMP-ISIJ Vol. 9 (1996), pp.1112-1115 the present inventors have reported on the high-speed deformation properties and impact energy absorption of high-strength thin steel sheets, and in that article it was reported that the dynamic strength in the high strain rate range of about 103 (l/s) is drastically increased in comparison to the static strength in the low strain rate of 10xe2x88x923 (l/s), that the strain rate dependence for deformation resistance varies based on the strengthening mechanism for the material, and that TRIP (transformation induced plasticity) steel sheets and DP (ferrite/martensite dual phase) steel sheets possess both excellent formability and impact absorption properties compared to other high strength steel sheets.
Furthermore, Japanese Unexamined Patent Publication No. 7-18372, which provides retained austenite-containing high strength steel sheets with excellent impact resistance and a method for their production, discloses a solution for impact absorption simply by increasing the yield stress brought about by a higher deformation rate; however, it has not been demonstrated what other aspects of the retained austenite should be controlled, apart from the amount of retained austenite, in order to improve impact absorption.
Thus, although understanding continues to improve with regard to the dynamic deformation properties of member constituent materials affecting absorption of impact energy in automobile collisions, it is still not fully understood what properties should be maximized to obtain steel materials for automotive members with more excellent impact energy absorption properties, and on what criteria the selection of materials should be based. Steel materials for automotive members are formed into the required part shapes by press molding and, after usually undergoing painting and baking, are then incorporated into automobiles and subjected to actual instances of impact. However, it is still not clear what steel-strengthening mechanisms are suitable for improving the impact energy absorption of steel materials against collisions subsequent to such pre-deformation and baking treatment.
It is an object of the present invention to provide high-strength steel sheets with high impact energy absorption properties as steel materials for shaping and forming into such parts as front side members which absorb impact energy upon collision, as well as a method for their production. First, the high-strength steel sheets exhibiting high impact energy absorption properties according to the present invention include:
(1) The press formable high-strength steel sheets with high flow stress during dynamic deformation, characterized in that the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength as when deformed in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the tensile deformation strength ad when deformed at a strain rate of 5xc3x97102xcx9c5xc3x97103 (l/s) after the aforementioned pre-deformation, i.e. "sgr"dxe2x88x92"sgr"s, is at least 60 MPa, and the work hardening coefficient between 5% and 10% of a strain is at least 0.130; and
(2) The press formable high-strength steel sheets with high flow stress during dynamic deformation, characterized in that the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength "sgr"s when deformed in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength ad when deformed at a strain rate of 5xc3x97102xcx9c5xc3x97103 (l/s) after the aforementioned pre-deformation, i.e. "sgr"dxe2x88x92"sgr"s, is at least 60 MPa, the difference between the average value "sgr"dyn (MPa) of the flow stress at an equivalent strain in the range of 3xcx9c10% when deformed in a strain rate range of 5xc3x97102xcx9c5xc3x97103 (l/s) and the average value "sgr"st (MPa) of the flow stress at an equivalent strain in the range of 3xcx9c10% when deformed in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s) satisfies the inequality: ("sgr"dynxe2x88x92"sgr"st)xe2x89xa7xe2x88x920.272xc3x97TS+300 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5xc3x9710xe2x88x924 2xcx9c5xc3x9710xe2x88x923 (l/s), and the work hardening coefficient between 5% and 10% of a strain is at least 0.130.
They further include:
(3) The press formable high-strength steel sheets with high flow stress during dynamic deformation, characterized in that the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength as when deformed in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength ad when deformed at a strain rate of 5xc3x97102xcx9c5xc3x97103 (l/s) after the aforementioned pre-deformation, i.e. "sgr"dxe2x88x92"sgr"s, is at least 60 MPa, the difference between the average value "sgr"dyn (MPa) of the flow stress at an equivalent strain in the range of 3xcx9c10% when deformed in a strain rate range of 5xc3x97102xcx9c5xc3x97103 (l/s) and the average value "sgr"st (MPa) of the flow stress at an equivalent strain in the range of 3xcx9c10% when deformed in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s) satisfies the inequality: ("sgr"dynxe2x88x92"sgr"st)xe2x89xa7xe2x88x920.272xc3x97TS+300 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s), the value (M) determined by the solid solution [C] in the retained austenite and the average Mn equivalents of the steel material {Mn eq=Mn+(Ni+Cr+Cu+Mo)/2}, defined by the equation M=678xe2x88x92428xc3x97[C]xe2x88x9233 Mn eq is at least xe2x88x92140 and less than 70, the retained austenite volume fraction of the steel material after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10% is at least 2.5%, the ratio between the initial volume fraction of the retained austenite V(0) and the volume fraction of the retained austenite after pre-deformation at an equivalent strain of 10% V(10), i.e. V(10)/V(0) is at least 0.3, and the work hardening coefficient between 5% and 10% of a strain is at least 0.130.
They still further include:
(4) The high-strength steel sheets with high flow stress during dynamic deformation according to any of (1)-(3) above wherein any of the following conditions are satisfied: the average grain diameter of the retained austenite is no greater than 5 xcexcm; the ratio of the average grain diameter of the retained austenite and the average grain diameter of the ferrite or bainite in the dominant phase is no greater than 0.6 while the average grain diamter of the dominant phase is no greater than 10 xcexcm and preferably no greater than 6 xcexcm; the volume fraction of the martensite is 3xcx9c30% while the average grain diameter of the martensite is no greater than 10 xcexcm and preferably no greater than 5 xcexcm; and the volume fraction of the ferrite is at least 40% while the value of the tensile strengthxc3x97total elongation is at least 20,000.
(5) The high-strength steel sheets of the present invention are also high-strength steel sheets containing, in terms of weight percentage, C at from 0.03% to 0.3%, either or both Si and Al at a total of from 0.5% to 3.0% and if necessary one or more from among Mn, Ni, Cr, Cu and Mo at a total of from 0.5% to 3.5%, with the remainder Fe as the primary component, or they are high-strength steel sheets with high flow stress during dynamic deformation obtained by further addition if necessary to the aforementioned high-strength steel sheets, one or more from among Nb, Ti, V, P, B, Ca and REM, with one or more from among Nb, Ti and V at a total of no greater than 0.3%, P at no greater than 0.3%, B at no greater than 0.01%, Ca at from 0.0005% to 0.01% and REM at from 0.005% to 0.05%, with the remainder Fe as the primary component.
(6) The method for producing high-strength hot-rolled steel sheets with high flow stress during dynamic deformation according to the present invention, for press formable high-strength steel sheets with high flow stress during dynamic deformation where the microstructure of the hot-rolled steel sheets is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite of the volume fraction between 3% and 50%, wherein the difference between the static tensile strength as when deformed in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength ad when deformed at a strain rate of 5xc3x97102xcx9c5xc3x97103 (l/s) after the aforementioned pre-deformation, i.e. "sgr"dxe2x88x92"sgr"s, is at least 60 MPa, the difference between the average value "sgr"dyn (MPa) of the flow stress at an equivalent strain in the range of 3-10% when deformed in a strain rate range of 5xc3x97102xcx9c5xc3x97103 (l/s) and the average value "sgr"st (MPa) of the flow stress at an equivalent strain in the range of 3xcx9c10% when deformed in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s) satisfies the inequality: ("sgr"dynxe2x88x92"sgr"st)xe2x89xa7xe2x88x920.272xc3x97TS+300 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s), and the work hardening coefficient between 5% and 10% of a strain is at least 0.130, is characterized in that a continuous cast slab having the component composition of (5) above is fed directly from casting to a hot rolling step, or is hot rolled after reheating, the hot rolling is completed at a finishing temperature of Ar3xe2x88x9250xc2x0 C. to Ar3+120xc2x0 C., and after cooling at an average cooling rate of 5xc2x0 C./sec in a cooling process following the hot rolling, the slab is coiled at a temperature of no greater than 500xc2x0 C.
(7) The method of producing high-strength hot-rolled steel sheets with high flow stress during dynamic deformation is also that described in (6) above, wherein at the finishing temperature for hot-rolling in a range of Ar3xe2x88x9250xc2x0 C. to Ar3+120xc2x0 C., the hot rolling is carried out so that the metallurgy parameter: A satisfies inequalities (1) and (2) below, the subsequent average cooling rate in the run-out table is at least 5xc2x0 C./sec, and the coiling is accomplished so that the relationship between the above-mentioned metallurgy parameter: A and the coiling temperature (CT) satisfies inequality (3) below.
9xe2x89xa6log Axe2x89xa618xe2x80x83xe2x80x83(1)
xcex94Txe2x89xa621xc3x97log Axe2x88x92178xe2x80x83xe2x80x83(2)
6xc3x97log A+312xe2x89xa6CTxe2x89xa66xc3x97log A+392xe2x80x83xe2x80x83(3)
(8) The method for producing high-strength cold-rolled steel sheets with high flow stress during dynamic deformation according to the present invention, for press formable high-strength steel sheets with high flow stress during dynamic deformation where the microstructure of the cold-rolled steel sheets is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite of the volume fraction between 3% and 50%, wherein the difference between the static tensile strength as when deformed in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength "sgr"d when deformed at a strain rate of 5xc3x97102xcx9c5xc3x97103 (l/s) after the aforementioned pre-deformation, i.e. "sgr"dxe2x88x92"sgr"s, is at least 60 MPa, the difference between the average value "sgr"dyn (MPa) of the flow stress at an equivalent strain in the range of 3xcx9c10% when deformed in a strain rate range of 5xc3x97102xcx9c5xc3x97103 (l/s) and the average value "sgr"st (MPa) of the flow stress at an equivalent strain in the range of 3xcx9c10% when deformed in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s) satisfies the inequality: ("sgr"dynxe2x88x92"sgr"st)xe2x89xa7xe2x88x920.272xc3x97TS+300 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s), and the work hardening coefficient between 5% and 10% of a strain is at least 0.130, is characterized in that a continuous cast slab having the component composition of (5) above is fed directly from casting to a hot rolling step, or is hot rolled after reheating, the coiled hot-rolled steel sheets after hot rolling is subjected to acid pickling and then cold-rolled, and during annealing in a continuous annealing step for preparation of the final product, annealing for 10 seconds to 3 minutes at a temperature of from 0.1xc3x97(AC3xe2x88x92Ac1)+Ac1xc2x0 C. to Ac3+50xc2x0 C. is followed by cooling to a primary cooling stop temperature in the range of 550xcx9c720xc2x0 C. at a primary cooling rate of 1xcx9c10xc2x0 C./sec and then by cooling to a secondary cooling stop temperature in the range of 200xcx9c450xc2x0 C. at a secondary cooling rate of 10xcx9c200xc2x0 C./sec, after which the temperature is held in a range of 200xcx9c500xc2x0 C. for 15 seconds to 20 minutes prior to cooling to room temperature; or
(9) The method described in (8) above for press formable high-strength steel sheets with high flow stress during dynamic deformation where the microstructure of the cold-rolled steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite of the volume fraction between 3% and 50%, wherein the difference between the static tensile strength as when deformed in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength "sgr"d when deformed at a strain rate of 5xc3x97102xcx9c5xc3x97103 (l/s) after the aforementioned pre-deformation, i.e. "sgr"dxe2x88x92"sgr"s, is at least 60 MPa, the difference between the average value "sgr"dyn (MPa) of the flow stress at an equivalent strain in the range of 3-10% when deformed in a strain rate range of 5xc3x97102xcx9c5xc3x97103 (l/s) and the average value "sgr"st (MPa) of the flow stress at an equivalent strain in the range of 3-10% when deformed in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s) satisfies the inequality: ("sgr"dynxe2x88x92"sgr"st)xe2x89xa7xe2x88x920.272xc3x97TS+300 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5xc3x9710xe2x88x924xcx9c5xc3x9710xe2x88x923 (l/s), and the work hardening coefficient between 5% and 10% of a strain is at least 0.130, characterized in that during annealing in the continuous annealing step for preparation of the final product, annealing for 10 seconds to 3 minutes at a temperature of from 0.1xc3x97(Ac3xe2x88x92Ac1)+Ac1xc2x0 C. to Ac3+50xc2x0 C. is followed by cooling to a secondary cooling start temperature Tq in the range of 550xcx9c720xc2x0 C. at the primary cooling rate of 1xcx9c10xc2x0 C./sec and then by cooling to a secondary cooling stop temperature Te in the range from the temperature Tem determined by the component and annealing temperature To to 500xc2x0 C. at the secondary cooling rate of 10xcx9c200xc2x0 C./sec, after which the temperature Toa is held in a range of Texe2x88x9250xc2x0 C. to 500xc2x0 C. for 15 seconds to 20 minutes prior to cooling to room temperature.